1. Field of the Invention
The present invention relates to a relay device, a wireless communication system and a multicast relay method that realize multicast communications efficient and effective in mobile communications.
2. Description of the Related Art
WiMAX (Worldwide Interoperability for Microwave Access) is defined as a fixed wireless communication technology based on IEEE802.16. Standardization of the WiMAX technology for providing a fast wireless communication service is in progress. The WiMAX Forum® is an industry-led organization established as an organization for promoting the WiMAX technology. Non-Patent document 1 given below is a document issued by the WiMAX Forum® as a technical overview about the WiMAX technology.
Non-Patent document 1 describes a purport of supporting a multi BS (Base Station)-MBS (Multicast Broadcast Service) in the WiMAX technology. Specifically, the wireless communication system using the WiMAX technology is requested to provide, through multicast communications, a variety of contents such as a news program, a sports program, a weather forecast and traffic information.
IGMP (Internet Group Management Protocol) and MLD (Multicast Listener Discovery) are employed as multicast group management protocols in a way that participates in and leaves a multicast group in the multicast communications using an IP (Internet Protocol) protocol. The IGMP is the multicast group management protocol that supports IPv4, and the MLD is the multicast group management protocol that supports IPv6.
The following document is a conventional art related document that discloses a system etc for supporting the multicast communications. Patent document 1 given below discloses a wireless multicast system etc. The wireless multicast system is that a router storing associative information between information representing wireless access point equipment that transmits the packet to the multicast group and information representing the multicast group, specifies the access point equipment that should establish a connection to a wireless terminal by transmitting the associative information to the wireless terminal.
Further, Patent document 2 given below discloses an IP packet multicast method. According to this method, in relay nodes having a hierarchical structure, a high-order node storing low-order node information in response to a participation request given from the low-order node and deletes the information in response to a cancellation request.
The Patent document 1 is a “Japanese Patent Application Laid-Open Publication No. 2004-15435”.
The Patent document 2 is a “Japanese Patent Application Laid-Open Publication No. 2002-94562”.
The Non-Patent document 1 is a “WiMAX Forum, “A Technical Overview and Performance Evaluation”, Mobile WiMAX-Part 1, Feb. 16, 2006”.
A wireless multicast system disclosed in Patent document 1 described above is not, however, suited to a system that supports mobile communications because of making the wireless terminal desiring to receive the packet of the predetermined multicast group select the should-be-connected wireless access point equipment.
Further, according to a multicast communication method disclosed in Patent document 2 given above, each of the base station (RNC) manages the information on the participating multicast group regarding each of the connected mobile terminals, and hence such a problem arises that the information managed in each base station increases in quantity and needs synchronizing between the respective base stations, resulting in intricacy of the system architecture.
Further, Non-Patent document 1 does not disclose a specific method of supporting the multi BS-MBS in the WiMAX technology.